1. Field of the Invention
The present invention relates to a voltage controlled oscillator circuit that is used for a phase locked loop circuit or the like. In particular, the present invention relates to a voltage controlled oscillator circuit that is capable of outputting a wide frequency range.
2. Description of the Prior Art
Conventionally, voltage controlled oscillator (VCO) circuits are used for generating a carrier or a clock signal or as an element constituting a phase locked loop (PLL) circuit in various communication devices such as a mobile telephone, computer-related devices and other various digital devices or control devices (see Japanese unexamined patent publication No. 2002-171165).
As shown in FIG. 11, a VCO 80 is basically made up of a ring oscillator 82 including a plurality of delay circuits 81a, 81b, . . . , 81n connected like a ring and a voltage to current converter circuit 83 for controlling current that flows in the ring oscillator 82 so as to alter its oscillation frequency.
The VCO 80 is required to generate little noise and have a high purity of signal. In particular, it is important to reduce a phase noise.
On the other hand, the VCO is also required to be capable of covering a frequency range that is as wide as possible. In order to realize this requirement, there are following methods.
A first method is used for covering a frequency range by one VCO. In this case, in order to output a clock signal having a wide frequency range, a gradient of input voltage versus output frequency characteristic (a VF characteristic) is set to a large value. In other words, a gain is set to a high value. More specifically, since a frequency f outputted by the VCO is proportional to input voltage Vctr as shown in FIG. 12, a frequency range can be enlarged by increasing the gradient of the frequency f versus the input voltage Vctr.
A second method is a method of enlarging a frequency range by connecting a divider to the output terminal of the VCO so as to switch a dividing ratio of the divider.
A third method uses a plurality of VCOs having different frequency bands and switches among the plurality of VCOs so that the frequency range is enlarged.
In addition, as a fourth method, it is proposed to use a current mirror circuit so as to restrict current that flows in each of a plurality of delay circuits 81 (see Japanese unexamined patent publication No. 7-58602). More specifically, in this method, a plurality of transistors having different sizes are used, and they are switched so as to control current that flows in each of the delay circuits 81.
In the case of the first method described above, the input voltage Vctr becomes lower in a low frequency range than the case where a gradient of the VF characteristic is small, so that current of the voltage to current converter circuit 83 also decreases. For this reason, there is a problem of deterioration of the phase noise characteristics.
More specifically, there is a random noise as well as a 1/f noise as a noise generated by a MOS transistor in general. The 1/f noise is dominant in the phase noise of the VCO. A magnitude of the 1/f noise is indicated by a square of gate voltage V as follows.V2=(K/( Cox·W·L))×(1/f),
where, K represents the Boltzmann constant, Cox represents a gate capacitance per unit area, W represents a gate width, L represents a gate length and f represents an output frequency.
Therefore, it is understood that the larger the values W and L of the transistor are, the smaller the 1/f noise is, if the current does not change.
On the other hand, this voltage is converted into a current noise as follows.i2=V2×gm2,
where, gm represents a conductance of a transformer.
Since the phase noise is considered to be proportional to i/Id, the current noise can be expressed as follows.
                              i          2                =                              V            2                    ×                      (                                          gm                2                            /                              Id                2                                      )                                                            =                                    V              2                        ×            2            ⁢                                                  ⁢            μ            ×                          Cox              ·                                                X                  ⁡                                      (                                          W                      /                      L                                        )                                                  /                Id                                                    ,            
where, μ represents a drift speed of carriers, Id represents drain current.
Therefore, it is understood that the larger the drain current is, the smaller the phase noise becomes, under the condition of the same values of W and L. In addition, the voltage to current converter circuit has higher sensitivity to the phase noise than the ring oscillator. In other words, the phase noise increases if current in the voltage to current converter circuit is decreased.
Therefore, in the case of the first method, the input voltage Vctr becomes low in a low frequency range. As a result, current in the voltage to current converter circuit is also decreased so that the phase noise increases.
In the case of the second method, there is a problem of large current consumption. Although it is expected inherently that the power consumption becomes smaller in a low frequency than in a high frequency, the power consumption becomes large in the second method since the VCO always operates at a high frequency.
In the case of the third method, there is a problem of large power consumption and a large area occupied by circuit elements since a plurality of VCOs exist.
In the case of the method disclosed in the second document (Japanese unexamined patent publication No. 7-58602), there is a problem of a complicated circuit and a large area occupied by circuit elements since each delay circuit needs transistors for the current mirror circuit.